Single-phase A-C variable speed controls for fractional and small integral horsepower motors are widely available in the prior art, and there are many designs for these controllers to achieve the variable speed control in this range of motor size. However, there are many applications which include a number of three-phase A-C motors and in which it would be desirable to provide variable speed control for a motor of the same range, except in three-phase. One such example is in a condenser for a large air conditioning installation in which there is one fan motor which is utilized to cool the condenser coils during the off season, that motor being of the same size as the other three-phase A-C motors used in the hot season as a greatly reduced amount of air flow is required and desired to maintain the condenser temperature at its optimum for efficient system operation. Generally, in those applications a small single-phase A-C motor and variable speed control is provided as there are few three-phase A-C variable speed controllers which are economically feasible for that application. As can be appreciated by one of ordinary skill in the art, it is much more desirable to utilize a three-phase motor and control in such an application as it simplifies the design of the system, eliminates the equipment necessary to convert three-phase to single-phase, and also utilizes three-phase motors throughout the system which can significantly reduce the inventory of motors required to manufacture and sell systems of this design.
To solve these and other problems, the inventor herein has succeeded in designing and developing a three-phase A-C speed control for variable torque motor applications in the small integral and fractional horsepower range. These applications include fan motors, pumps, blowers, and other applications in which the motor can operate through a wide slip range. The controller of the present invention provides approximately six-to-one speed control, and has many unique features which simplify the design, and which uniquely generate and synchronize the trigger signals utilized in switching on the three motor windings in proper sequence and phase delay. The speed control of the present invention includes at its heart a phase locked loop including a phase comparator which compares the phase delay between a single phase 60 Hz. reference signal and a feedback signal from a voltage controlled oscillator circuit operating at 5760 Hz. whose output is reduced to 60 Hz. by a two-stage divider circuit. The speed control signal is injected into a low pass filter which connects the phase comparator with the voltage controlled oscillator so that as the desired speed is changed, the phase of the feedback signal generated by the voltage controlled oscillator is changed, and the output of the phase comparator is changed accordingly. A D-type flip-flop is connected to the output of the phase comparator, and is clocked by the 5760 Hz. generated by the voltage controlled oscillator to produce a stream of pulses at 120 Hz. having a delay corresponding to the phase delay between the 60 Hz. feedback signal and the 60 Hz. reference signal. This 120 Hz. pulse stream is used to trigger a first winding of the three motor windings. This first pulse train is also input to a pair of 16 bit shift registers which are connected in cascade which, because of the VCO frequency, provide a 60.degree. delay between their outputs which are also 120 Hz. pulse trains. An electronic switching circuit comprised of logic gates and triacs, and their associated isolation circuitry are utilized to switch A-C power to the motor windings.
As the speed controller is comprised of electronic components, it can be mounted to a PC board with suitable heat sinking and be packaged for connection to the three motor leads, the three A-C voltage leads, and two input leads for the control signal. This controller will thus provide variable speed control with a small three-phase A-C motor. The principal advantages and features of this invention have been mentioned above. However, a greater understanding of the invention and a fuller appreciation of the full range of features provided by this invention can be achieved by referring to the drawings and Detailed Description of the Preferred Embodiment which follows.